In association with recent spread and improvement in performance of mobile electronic devices, an electrochemical element with high energy density has been desired. As an electrochemical element of this type, a lithium ion secondary battery using an organic electrolyte (non-aqueous electrolyte) has gathered attention. An average voltage of the lithium ion secondary battery is 3.7 V, which is approximately three times of an average voltage of an alkaline secondary battery, and the lithium ion secondary battery has such high energy density. However, since the lithium ion secondary battery cannot use an aqueous electrolyte as the alkaline secondary battery does, it uses a non-aqueous electrolyte with sufficient oxidation-reduction resistance.
A capacitor, which is another kind of electrochemical element, has large electric capacity as well as high stability against repeated charges and discharges. Accordingly, it is widely used for an application to a power feeding source or a similar device used for vehicles and electrical devices.
Among the above-described separators for electrochemical element, a film-shaped porous film made of polyolefin is often used as a separator for lithium ion secondary battery (for example, see Patent Document 1). However, this separator for lithium ion secondary battery has low electrolyte-retaining ability, and therefore has low ion conductivity, causing a problem of high internal resistance.
Furthermore, as a separator for lithium ion secondary battery, nonwoven separators made of synthetic fiber have been proposed (for example, see Patent Documents 2˜4). However, these separators have the following problems. Due to low electrolyte-retaining ability, the internal resistance becomes high. Moreover, since the separators have insufficient denseness, they cause high proportion defective in internal short-circuit and poorness of high rate characteristics, discharge characteristics, and their variations.
As separators for lithium ion secondary battery, there have been proposed a separator made of fibrillated heat-resistant fiber, fibrillated cellulose, or non-fibrillated fiber; and a separator made of fibrillated solvent spun cellulose fiber or synthetic fiber (for example, see Patent Documents 5 and 6). However, these separators yet have room for improvement in strength thereof.
As the separator for capacitor, a separator made of paper mainly containing beaten solvent spun cellulose fiber or beaten regenerated cellulose fiber has been used (for example, see Patent Documents 7˜9). Nowadays, for thinning the separator for capacitor, there has been proposed a separator formed by laminating two or more fiber layers (for example, see Patent Document 10). However, since this separator is formed by laminating a layer made of synthetic fiber and a layer made of a fibrillated solvent spun cellulose fiber, adhesive strength between the layers is insufficient, which is likely to cause a problem of delamination.
Actually, to incorporate the separator for electrochemical element into the electrochemical element, the separator needs to be cut as necessary. However, if the cutting processability of the separator and the quality of the cut section of the separator are poor, fiber naps generated at the cut section and fallen fibers may deteriorate characteristics of a final electrochemical element.